1. Field of the Invention
The present invention relates to a vitreous silica crucible and a method of manufacturing a silicon ingot.
2. Description of Related Art
In general, silicon single crystal is manufactured by melting high-purity polycrystalline silicon in a vitreous silica crucible to obtain silicon melt, dipping an end of a seed crystal to the silicon melt, and pulling the seed crystal while rotating it.
The melting point of silicon is 1410 deg. C, and thus the temperature of silicon melt is kept at a temperature higher than 1410 deg. C. At such temperature, a vitreous silica crucible reacts with silicon melt, and the thickness of the crucible wall gradually decreases. When the thickness of the crucible wall decreases, the strength of the crucible is lowered. This leads to problems such as buckling and sidewall lowering of the crucible.
Furthermore, island-like crystals are formed on the crucible inner surface by the reaction between the crucible and the silicon melt. These island-like crystals are called “brown rings” because the periphery of the island-like crystal is brown. The center portion of the brown ring is thin, and thus the center portion of the brown ring can be peeled off from the crucible inner surface when the thickness of the brown ring is further reduced by corrosion caused by reaction between the crucible and silicon melt. When the peeling occurs, there occur problems such as the peeled crystal pieces are mixed in a silicon ingot, and newly generated small crystals (the same constituent as the brown ring) are peeled off and mixed in a silicon ingot, which leads to deterioration of crystallinity of a silicon ingot.
In order to solve these problems, there is known a technique to add alkali earth metal on the crucible inner surface to crystallize the crucible inner surface. The crystallization enhances the crucible strength and suppresses generation of brown rings (See Japanese Patents 3046545 and 3100836).